A product I can't remember if I've mentioned before,

and if I haven't I should have.  Blue Wonder Gun Cleaner.

You may have noticed I like to try stuff; ran across this a few years ago, and it works well.  It's a thin gel.  Process is

Set your gun where it'll be stable.

Put a good amount on a bore brush.

All the way through, back & forth, five or six times.

Repeat.

Let set a while.

Brush again.

Patches and a jag to clean it out.
Oil.

The stuff, is real good at working under bad fouling so the brush can break it loose and the patches push it out.  And you can leave it for a few hours without damage if the bore is really bad.  It works pretty well.

Has another use: for surface rust, put some on a piece of #0000 steel wool and rub lightly, it takes the rust off without bothering the finish around.  Then clean off completely, and whatever oil/clp you use.

I do recommend it.  They've got a number of other products, I haven't tried those.  Maybe someday.